Genetics in ophthalmology / Arnold Sorsby.

  • Sorsby, Arnold, 1900-1980.
Date:
1951
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics in ophthalmology / Arnold Sorsby. Source: Wellcome Collection.

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    SOME GENERAL CON^flPTS Linkage of two genes on the X chromosome produces a situation somewhat different from that with an autosome. The theoretical possibihties are shown in Fig. 13. If the two genes are carried on the non-homologous part of the X chromo¬ some of a man (Fig. 13 я) the situation does not differ in any way from that seen if only one gene were so carried. Whether recessive or dominant they would manifest themselves in the man and would be transmitted to all his daughters, who would not be affected unless the genes were dominant. If on the other hand the genes are carried on the X chromosome of a woman, two situations arise. If both genes are carried on one and the same X chromosome (Fig. \ЪЬ) the mother will be unaffected if the genes are recessive, and will show both affections if dominant; 50 per cent of her sons and, in the absence of cross-over, 50 per cent of her daughters will likewise carry both genes. The daughters will show the two 0 0 0 0 0 0 9 Ml 0 0 e («) Ф) {с) Fig. 13.—Inheritance of two pathogenic genes carried on the non-homologous part of the X chromosome. (a) The father carries the pathogenic gene on the X chromosome and is affected. The mother is normal. All daughters will carry both genes and will be affected only if the genes are dominant. The sons are normal. {b) The mother carries two hnked pathogenic genes and will be affected if the genes are dominant. The father is normal. Half the sons (carrying both genes) will be affected whether the genes are dominant or recessive. Half the daughters will carry the gene and will be affected only if the genes are dominant. (c) The mother carries the two pathogenic genes, one on each of the two X chromosomes, and will be affected only if the genes are dominant-. The father is normal. All the children will carry one or other of the genes. The daughters will be affected only if the genes are dominant. All the sons will be affected with one or other abnormality regardless of whether the genes are dominant or recessive. affections only if the genes are dominant, whilst the sons will show the two affections irrespective of whether the genes are recessive or dominant. Should the genes be carried by the mother, one on each X chromosome (Fig. 13 c), she would once more show both affections if the genes were dominant and neither if the genes were recessive. All her children will, however, be carriers. The daughters would be affected with one condition only if the gene were dominant, and would be altogether unaffected if the gene were recessive. The sons would show one or other affection only, irrespective of whether the gene were recessive or dominant. 28